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Table Of Contents
Calculating Riser Values
533
Riser Equations and Steps Used to Calculate Riser Values
The following equations and steps are used by the Riser Calculator to calculate
values.
Equations used: (From Chapters 3 and 6 in the SCS Engineering Field Manual)
Friction flow: Q
f
= (1.486 / n) * R
h
2/3
* Slope
1/2
* Area
Head flow: Q
h
= ((2 * G
a
* PipeHead) / (1 + K
e
+ K
b
+ K
p
* Length))
1/2
Weir flow: Q
weir
= WeirCoefficient * PI * Diameter * RiserHead
3/2
Orifice flow: Q
orifice
= OrificeCoefficient * (Diameter
2
/ 4) * PI * (2 * G
a
*
RiserHead)
1/2
where:
K
p
= (5077 * n
2
) / Diameter
4/3
Pipe friction coefficient. Diameter is in inches.
K
b
= (Bend * n) / 3 (minor loss due to bend).
K
e
= Entrance Coefficient (minor loss due to entrance).
Area = Area of pipe (use full) (ft
2
).
Length = Length of pipe (ft).
n = Mannings roughness coefficient for pipe.
R
h
= Hydraulic radius (wetted area / wetted perimeter  use full flow in this
case) (ft).
G
a
= Acceleration due to gravity 32.174 (ft/sec
2
).
Bend = User input. Range 0-90 degrees (0 for no bend).
Slope = Slope of Pipe (elevation difference / length) (ft).
PipeHead = Spillway elevation  Outlet elevation (ft).
Diameter = vertical pipe diameter from table (ft).
RiserHead = Elevation difference between rim of vertical pipe and the water
surface (ft).
WeirCoefficient = User input (usually 3.33).
OrificeCoefficient = User input (within range of 0.50.83).
PI = 3.14159.
For more information on the steps used to calculate riser values, see Autodesk
Civil Design online Help.